1. Field of the Invention
This invention relates to magnetic particles and particularly to magnetic particles having a coating thereon suitable for use in a magnetic recording layer particularly on a photographic element the magnetic recording layer being transparent in a photographic sense.
2. Description of Related Art
Conventional magnetic recording elements that are used for recording sounds or images are generally opaque to visible light regardless of the nature of the magnetic particles used in such elements. For example, motion picture films often are provided with a magnetic sound track which generally is opaque and does not cover that portion of the film used in the projection of images.
U.S. Pat. No. 4,280,918 discloses magnetic particles provided with a uniform coating of colloidal silica to prevent the aggregation of the magnetic particles in the magnetic coating mixture.
U.S. Pat. No. 4,336,310 discloses a substrate coated with a magnetic layer comprising a binder resin and a metallic magnetic powder supporting silica on the surface coated with sequential layers of hydrolyzed silane compound and oleic acid.
Canadian Patent 686,172 shows that a magnetic recording layer may be transparent to visible light when it contains low concentrations of magnetizable particles. According to this patent, such a layer is coated over a layer containing descriptive material which allows a user to simultaneously hear and see certain subject matter. However, this patent points out that the electromagnetic characteristics, i.e., the magnetic recording and reproducing characteristics, of such a layer are inferior to those of conventional magnetic layers as a result of the very low concentration of magnetizable particles.
U.S. Pat. No. 3,782,947 discloses a photographic product which carries magnetic particles distributed across the image area of the product. The particle distribution and sizes are so designed that the composite granularities of the photographic and magnetic distribution is essentially transparent in a photographic sense. According to this patent, the photographic image can be viewed via the magnetic distribution and the magnetic distribution can be employed for recording and playback information.
U.S. Pat. No. 4,279,945 and 4,302,523 disclose a process of preparing magnetic recording elements containing a transparent recording layer. According to these patents, the magnetic recording and reproducing characteristics obtained are comparable to conventional opaque magnetic layers without the need for matching the granularity of a magnetic medium to that of a photographic medium. However, the process requires that the layer containing magnetic particles be treated using one or both of the following process steps, (1) compacting the layer while it is in a malleable state to reduce its thickness (e.g., calendaring), or (2) imbibing into the layer a substantially transparent liquid having a refractive index that is substantially the same as that of the binder.
Elements of the type described in the above-cited patents have not achieved widespread commercial success for various reasons. For example, the elements described in U.S. Pat. No. 4,279,945, as indicated by the Figure therein, are substantially opaque at wavelengths less than about 500 nm and thus are not useful in color films. Further, the disclosed process requires that the magnetic recording layer be calendered while it is in a malleable state and/or that a transparent liquid be imbibed into the magnetic recording layer.
It is evident that it would be highly desirable to provide magnetic particles that when incorporated into a transparent binder will provide a layer of film that is transparent in the photographic sense but also capable as a magnetic recording layer exhibiting improved magnetic and photographic performance. This goal is difficult to achieve because of the nature of the magnetic particles and the concentration of particles required to provide a sufficient signal to write and read magnetically stored data. There may also be noticeable color and haze associated with the magnetic layer depending upon the type of pigment, the concentration of the pigment and the effective particle size thereof.
The critical photographic properties affected by the magnetic layer are the optical density and the granularity. To reduce the impact of the magnetic layer on photographic quality, both the optical density and the granularity must be minimized so these properties have no adverse effects on the color, the brightness of highlighted areas and the granularity of prints made from negatives or projected images from transparencies. This is what is meant when layers are spoken of herein as being "transparent in a photographic sense".
In co-pending application Ser. No. 473,494, filed Feb. 1, 1990 by Robert O. James and John Rieth, and entitled "Photographic Element Containing Thin Transparent Magnetic Recording Layer and Method For the Preparation Thereof" is disclosed and claimed photographic elements containing a transparent magnetic recording layer wherein the magnetic particles included in the recording layer are cobalt surface treated gamma iron oxide particles having a specific surface area of at least 30 m.sup.2 /g in a concentration of the magnetic particles being from about 10 to about 1000 milligrams/m.sup.2.